The invention generally relates to electronic circuits that simulate passive circuit devices such as capacitors. More specifically, the invention relates to an electronic circuit and method for simulating a capacitor such as a fuel tank sensor capacitor.
A known technique for determining fluid levels in tanks such as aircraft fuel tanks is to place in the tank a series of capacitive sensors that have capacitance values that vary with the percentage of the capacitor plates covered by the fuel. Such systems are shown, for example, in U.S. Pat. Nos. 4,968,946 and 4,908,783 both issued to Maier. Each sensor comprises essentially a two-plate capacitor that is connected to a capacitance detection circuit. The capacitance is detected by applying a time varying voltage to the capacitor and measuring the resultant current. As is well known, the current in a capacitor is proportional to the time rate of change of the voltage across the capacitor. The proportionality constant is defined as the capacitance, C, or I=CdV/dT.
Because the fuel tank sensor capacitors are physically located in the aircraft fuel tanks, the capacitors are tested using test circuits that can be connected to the capacitor leads independently of the on-board electronics. The capacitors can thus be tested without the need to remove the sensors from the tanks for troubleshooting. However, failure isolation requires that technicians be able to determine that the test equipment is functional, as well as being able to check the integrity of the aircraft on-board electronics. Therefore, having access to a simulated capacitor that has a known value can provide a valuable troubleshooting tool that avoids the need to tear down the fuel tank before the exact cause of failure can be isolated.
Several factors should be considered for electronically simulating a fuel tank capacitor. For example, the circuit should simulate the fact that a capacitor functions as a high output impedance current source. This simulation may be required because the test circuits and on-board electronics may connect the capacitors to an operational amplifier virtual ground. Therefore, the simulator should provide an output current that is independent of an applied load. Another characteristic of a capacitor is that the measured capacitance is a function of the frequency of the applied excitation voltage. This characteristic results from a device parameter called the dissipation factor. The dissipation factor of a capacitor is defined as the ratio between the capacitor's equivalent series resistance and the capacitive reactance. At low excitation frequencies, for example 1 kilohertz, the dissipation factor is typically small. But as the excitation frequency increases, the dissipation factor also increases and is manifested as measuring a different value of capacitance for the device under test as compared to the lower frequency measurement. Furthermore, when the applied excitation voltage is sinusoidal, the dissipation factor can result in less than a ninety degree phase shift between the applied voltage and the resultant current.
Known attempts to simulate a capacitor electronically are not suitable for use as a fuel tank capacitor simulator. For example, U.S. Pat. No. 4,644,306 issued to Kleinberg describes a capacitor simulator that operates at high frequency and is intended to be used in an oscillator circuit. The simulated capacitor, however, apparently only provides a low impedance output from a voltage follower without simulating dissipation factor. U.S. Pat. No. 4,785,250 issued to Lawton also apparently does not provide a high output impedance and also does not simulate dissipation factor. This simulated capacitor also operates from an applied excitation current.
Accordingly, it is an object of the present invention to provide an electronic circuit and method of using the same for simulating a fuel tank sensor capacitor that utilize a high output impedance current source and that can be used to simulate dissipation factor. In a broader sense, it is an object of the present invention to provide an electronic circuit and method for simulating a capacitor that produce a current source output that is proportional to the rate of change of an applied excitation signal. Further objects of the invention are to provide an electronic capacitance simulation that can be adjusted for different values of simulated capacitors, that can be compensated for different excitation frequencies and that is insensitive to an applied load.